The present invention relates to an apparatus for influencing a light beam with one or more color filters, and more particularly to improvements in apparatus for preferably infinitely varying the positions of the constituents of one or more composite filters with reference to a beam of copying light in a photographic copying machine or the like.
It is already known to influence the beam of copying light in a photographic copier by one, two or more filters each of which is assembled of several portions or sectors movable relative to each other as well as relative to the beam of light in order to exert a more or less pronounced influence upon the beam of light which is about to reach the original. As a rule, each composite filter is assembled of at least three sectors and the various filters are disposed one behind the other, as considered in the direction of propagation of copying light. The filter sectors have edge faces which extend substantially radially from the center of the opening for the passage of copying light toward the original. As a rule, the sectors of a composite filter are movable to, or close to, the positions of exact registry so that they form a composite filter which can intercept all or nearly all of the light in a particular color. Reference may be had to German Pat. No. 24 11 301 which discloses an apparatus of the above outlined character wherein the sectors of three filters are pivotable about axes extending in parallelism with the direction of propagation of copying light. All sectors of a composite filter are pivotable or turnable by a common driving unit including pulleys, which are secured to the pivots for the sectors, and a cable or cord which is trained over the pulleys. The cable or cord is an endless flexible element which is further trained over a driver pulley serving to change the angular position of each pulley to the same extent in response to rotation of the driver pulley about its own axis. In other words, the extent of penetration of individual sectors forming a composite filter into or from the path of copying light is always the same. The driver pulley is rotated by hand; this is a time-consuming operation so that the patented apparatus is evidently not suited for use in modern and relatively complex copying machines which are designed to turn out large numbers of copies per unit of time. As a matter of fact, the patented apparatus can be used only and alone in very simple copying machines wherein the output is of secondary importance or of no importance at all. Attempts to rotate the driver pulley by a motor have met with failure because the rate at which the filters can influence the color of copying light does not vary linearly with the rate at which the filter sectors extend into the path of copying light. Due to its remanent light transmissivity, a filter sector which intercepts 10% of copying light does not ensure a 10% coloration of the light beam. On the other hand, the geometry of the filters is such that, shortly prior to at least substantially complete closing of a composite filter, its filtering action increases at a very rapid rate, i.e., a curve denoting the progress of light interception in a particular color is relatively flat during the initial stage or stages of introduction of filter sectors into the path of copying light, but the slope of such curve increases rapidly during the last stage or stages of movement of filter sectors to those end positions in which the respective composite filter intercepts all or nearly all of the light in a particular color.